The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for forming improved refractory metal layers having reduced stress while providing good gap filling properties.
Deposition of refractory metals, such as tungsten, over a semiconductor substrate is a common step in the formation of some integrated circuit (IC) structures. For example, tungsten is commonly used to provide electrical contact to portions of a semiconductor substrate. These electrical contacts are usually provided through openings in an insulation layer, such as a silicon dioxide layer, formed over the substrate. One method used to form such contacts includes the chemical vapor deposition (CVD) of tungsten to fill the opening after an initial layer of titanium nitride has been deposited in the opening. As another example, tungsten is sometimes used to form metal lines over a semiconductor substrate.
One CVD technique that has been employed to deposit tungsten films in the semiconductor industry uses tungsten hexafluoride (WF.sub.6) and a hydrogen reducing agent, e.g., H.sub.2, as precursor gases. This technique includes two main steps: nucleation and bulk deposition. The nucleation step grows a thin layer of tungsten which acts as a growth site for subsequent film. In addition to WF.sub.6 and H.sub.2, the process gas used in the nucleation step of this technique includes silane (SiH.sub.4), and may also include nitrogen (N.sub.2) and argon. A bulk deposition step then is used to form the tungsten film. The bulk deposition gas is a mixture containing WF.sub.6, H.sub.2, N.sub.2, and Ar.
Advances in integrated circuit technology have lead to a scaling down of device dimensions and an increase in chip size and complexity. This has necessitated improved methods for low temperature deposition of refractory metals, particularly tungsten, to enhance the gap filling properties and reduce the stress of the same. For purposes of this application, low temperature deposition is defined as a deposition process that occurs at temperatures no greater than 400.degree. C. Traditionally, the gap filling property and the stress are two characteristics of refractory metal layers that have been in conflict. For example, using prior low temperature deposition techniques, refractory metal layers having stress less than 1.5.times.10.sup.10 dynes/cm.sup.2 have been formed; however, the gap filling properties of these layers have been limited to less than 70%. Alternatively, refractory metal layers having gap filling properties greater than 90% have been formed using the aforementioned prior art deposition techniques. These layers, however, typically exhibit stress much greater than 1.5.times.10.sup.10 dynes/cm.sup.2.
What is needed, therefore, is a low temperature deposition process that enables rapid formation of refractory metal layers having reduced stress and superior gap filling properties.